Referring specifically to iron ore, iron is the world's most commonly used metal “steel” of which iron ore is the key ingredient, representing almost 95% of all metal used in a year.
Iron ore is the raw material used to make pig iron, which is one of the main raw materials used to make steel. 98% of the mined iron ore is used to make steel. In fact, iron ore is more integral to the world's economy than any other commodity except oil/fuel.
The principle ores of iron are hematite (70% iron) and magnetite (72% iron). Ore is typically found in magnetite and hematite minerals. Hematite is an iron oxide as is magnetite.
Initially, iron ore was predominately mined from hematite deposits with grades in excess of 60% iron. Such deposits are commonly referred to as “direct shipping ores” or “natural ores”. With depletion over time of high grade hematite deposits, lower grade ores have been further developed. This development of lower grade iron ores has led to the development of taconite iron ore.
Taconite is a “low grade” iron ore, only containing about 30% magnetite/hematite. This taconite iron ore comprises only small particles of magnetite and hematite interspersed with a very tough variety of quartz—chert. Chert is an extremely hard abrasive mineral.
To measure, in a relative way, the hardness of various materials such as chert, magnetite, hematite and other minerals, the Mohs scale, a relative measurement tool, was created in 1812 by the German geologist and mineralogist Friedrich Mohs. The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material.
The basic premise behind the Mohs scale is that, in order to identify (or bracket) the hardness (abrasion resistance) of a material an attempt is made to scratch a sample of the material with various materials listed on the Mohs scale of materials, until a particular material on the Mohs scale is identified that will scratch the sample of unknown material, while the Mohs scale “control” material is not scratched.
This same premise applies to truck bodies. As long as the material forming the truck body is higher on the Mohs scale than the material being hauled, there will be little to any abrading away (abrasion) of the load containing surfaces of the truck body. However, if the material being hauled in the truck body is higher on the Mohs scale than the material forming the truck body, then the floor, side plates, etc. of the truck body will be worn away (abraded) with every load hauled.
The original Mohs scale comprised the hardness/abrasion resistance numbers 1 thru 10, with 1 being the softest and 10 being the hardest, as shown in Table 1 below. Alternative Vickers hardness numbers (HV) (another way of comparing relative material hardenesses) are also shown by comparison.
TABLE 1Mohs scale of mineral hardnessMineralMohs HardnessVickers HVTalc1 20 HVGypsum2 70 HVCalcite3 110 HVFluorite4 180 HVApatite5 500 HVOrthoclase6 720 HVQuartz71280 HVChromium Carbides—1500 HVTopaz81620 HVMolybdenum Carbides—1800 HVCorundum92000 HVTitanium Carbides—3000 HVDiamond10 9000 HV
Over time the ten items first identified in the Mohs scale, which really only denotes relative hardness, required expanding to a more defined comparative hardness level and the Vickers scale was introduced to more clearly delineate relative hardness of materials.
The hardness of a material is further defined as the material's resistance to another material penetrating the material's surface and is related to the material's abrasion/wear resistance and strength. Higher hardness is generally related to higher strength, which in turn is related to the material's structure.
The Vickers hardness is defined by test standards, and requires a square pyramid indenter made of diamond to be pressed into a test sample at a specified load. The resulting indentation is then measured from tip to tip in both axes. The average measurement is converted to a Vickers hardness value according to a formula or a chart based on the formula.
Table 2 is a comparison between the Mohs number for various materials and a Vickers hardness number.
TABLE 2Comparison of Mohs hardness and Vickershardness for various materialsMineral NameHardness (Mohs)Hardness (Vickers) kg/mm2Graphite  1-2VHN10 = 7-11Tin1½-2VHN10 = 7-9Bismuth    2-2½VHN100 = 16-18Gold2½-3VHN10 = 30-34Silver2½-3VHN100 = 61-65Chalcocite2½-3VHN100 = 84-87Copper2½-3VHN100 = 77-99Galena2½VHN100 = 79-104Sphalerite3½-4VHN100 = 208-224Heazlewoodite4VHN100 = 230-254Carrollite  4½-5½VHN100 = 507-586Goethite    5-5½VHN100 = 667Hematite  5-6VHN100 = 1,000-1,100Chromite  5½VHN100 = 1,278-1,456Anatase5½-6VHN100 = 616-698Rutile    6-6½VHN100 = 894-974Pyrite    6-6½VHN100 = 1,505-1,520Bowieite1VHN100 = 858-1,288Euclase  7½VHN100 = 1,310Chromium9VHN100 = 1,875-2,000
Referring specifically to iron ore, on the Mohs scale and on the Vickers hardness scale, the basic components of taconite iron ore are shown in Table 3.
TABLE 3Mohs hardness and Vickers hardnessfor components of taconite iron oreMineral NameHardness (Mohs)Hardness (Vickers)Hermatite5.5 to 6.5+1,000 to 1,100Magnetite6.5+1,100Quartz (chert)71275
Referring to the steels used in producing truck bodies, typical high strength, high hardness steel used today in producing truck bodies has a basic maximum hardness of 450 Brinell. (The Brinell scale being the industry standard for hardness calculations of steel in this field.) In some highly abrasive truck body applications, a high hardness 600 Brinell steel or higher with minimal structural strength characteristics can also be used.
Having introduced Brinell along with the previously described Mohs scale and Vickers scale, it is important to understand the relationship between the following three material measurements:
1. Mohs Scale
2. Vickers
3. Brinell
The Vickers HV number is determined by the ratio F to A where “F” is the force applied to the material, and “A” is the surface area of the resulting indentation. The Vickers is used as an alternative to the Brinell method, and is different than the Mohs hardness scale which tests a material's scratch resistance.
The Brinell hardness scale is very similar to the Vickers hardness scale and was developed by a Swedish engineer named Johan August Brinell in 1900. The Brinell test utilizes a 10 mm diameter steel ball as an indenter, applying a uniform 3,000 kgf (29 kN) force. (A smaller amount of force is used on softer materials, and a tungsten carbide ball is used for harder materials.)
With the Mohs scale, the hardness of a sample is measured against standard test materials by finding the hardest material the sample can scratch, and/or by identifying the softest test material that can scratch the sample. For example, if a given sample can be scratched by quartz (Mohs 7) but not by topaz (Mohs 8), the sample has a hardness between 7 and 8 and is possibly about 7.5 on the Mohs scale.
TABLE 4Comparison of Vickers and BrinellBrinell HardnessVickers Hardness10 mm tungsten carbide ball—Load 3000 kg940—920—880(767)840(745)800(722)760(698)720(670)690(647)670(630)650611630591610573590554570535550517520488500471480452460433440415420397
It is readily apparent from Table 4 that the 450 Brinell high strength, high hardness steel commonly used in the manufacture of truck bodies has (only) a Vickers number of 480 and from Table 2 a corresponding Mohs scale number of about 4.75 which is far below the Vickers number of 1,000 to 1,275 (see Table 3) of hauled hard rock mineral ores such as iron ore and/or similar hard rock ores.
And, just as iron ore has a higher Vickers number than that of the high strength/high hardness steels commonly used to produce a typical truck body, there are many other mined hard rock ores that also have higher Vickers numbers than those of the steels used to produce conventional truck bodies.